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 Friday  |  03.29.2024
Products - behold Classic - Power Amplifier - BPA768 - Principels 

Principles of Developement of the BPA768:

Two Mono Blocks

The behold final stages BPA768 are built up very compact. They contain two completely independent mono blocks.

as if they were assembled of two final stages a la Classe Omega Mono Power amplifiers. Those final stages perform mono for example Classe 1000W/4Ω. (Manufacturers data, no detail about CW-rating is given)..

www.classeaudio.com

Or Halcro dm58 und dm68. We have tested the dm58 and measured the 400W/4Ω (Manufacturers data, CW-rating, we confirmed those data).

www.halcro.com

The behold BPA768 is built up completely symmetrical with its two mono blocks. The final power stages with their compact high performance cooling bodies and necessary power supplies are arranged left and right. The mono blocks themselves are constructed with two half bridge circuits resulting in a fully symmetrical full bridge circuit. This applies not only for the schematic design but also for the mechanical construction.


Central Processing:

The Central Processing is carried out from a central board (BPA-DSP). The board supplies with its signal processors the power supply section as well as the D/A-Converter of the final stages with all necessary control signals etc. The "music" is made in the D/A-Converters directly on the final stage circuit board (BPA-FPA) and, to be more precise, with the shortest possible track conductors of only a few centimeter and completely without any cable wiring connection.


The DC Coupling

By means of Digital Technique

there are no coupling condensers from the beginning of the signal source to the end of the analogue final stage amplifier output. The complete behold chain by that is dc coupled, also in the final stages.

No Lower Cut Off Frequency

The problems with the lower cut off frequency otherwise usual are consequently discontinued. There are no unwanted phase shifts and also no distortions always caused by coupling capacitors.

Special Care

is, however, required for the offset adjusting of the D/A-Conversion, the transimpedance and the final stage amplifier.


Using MOS-FETs

The Power Transistors

are FETs from IRF (IRF540 + IRF9540) and due to the processor technology we need only 2x 4 pieces per half bridge, therefore 16 per output. These transistors have a very simple case and transport their heat directly to the cooling block with lowest possible thermal resistance. This is possible because they are mounted direct on a copper block without any kind of isolation. The feeding of the voltage is using the same path as the heat.

The Selection of Transistors

We don't have any problem while selecting transistors. And we don't even need the emitter- / source-resistors. All other manufacturers are never thinking of that possibility ...


No Thick Cu-Rails

The Output Coupling

At behold the load currents do not need extra, sometime very stupid, bus conductors. In many other power amplifiers the output coupling is led to the bus conductor via PCB track conductors. This is often nonsense because many times they are designed much too small and only in one layer of the circuit board. At BPA768 this is completely different. Three bus conductors 27mm wide with extra thick copper in three layers of the circuit board transfer the signals and this is carried out separate for each half bridge.

The Feedback

is required for all power amplifiers of this construction. At the behold BPA768 that feedback to the transimpedance amplifier is not taken on the circuit board but on the output jack itself. By that all other possible negative influences on the path from the power transistors to the output jack are eliminated as far as technically feasible. For a better handling therefore plug connectors can be used here unhesitatingly.


No Relay?

Relay?

At behold "relay" should actually be an unknown term. This is unfortunately not the case, however, at behold BPA768 as the appropriate norms stipulate a mechanical separation of the mains voltage. It is unfortunately unavoidable if every thing shall be operated by the pre-amp APU768.

Semiconductors

cannot be used as they do not fulfil the today's requirement of the CE-testing. It therefore "clicks" are heared while turning on and off, but very quiet.

In the Analogue Signal

path are, however, no relays. The final amplifier is always connected to the loudspeaker. The noise of the on and off switching is reduced to a minimum by special transistor circuits. It is unfortunately not possible to avoid it completely.


RF-Blocking

No „Triple-by-passing“

We don't need, from Madrigal so-called capacitor circuits. There are only 20 pcs of 10.000µF capacitors and on the feeding point of the final amplifier transistors two of 1.0µF ceramic capacitors on each half bridge. In no case foil condensers are used as these would make resonance effects, i.e. exactly what is described correctly by Madrigal.

From the Radio Frequency Engineering

As our base is the radio frequency measuring techniques we can judge this for certain very well. We therefore do not need to simulate it but can measure it under real operating conditions. The results are of course different and much better. Simulations have their limits ...

Transient Problems

of course don't exist as long as they are not part of the music. In that case they have to be reproduced too. It's because the D/A-Conversion is arranged directly in front of the final stages and thus optimal coupled. An on-swing or resonance and the like are impossible with that.


200,000µF Only

Because of signal processor

controlled performance power supply the BPA768 needs only hardly imaginable small 200,000µF electrolytic capacitors in the +105°C version. The resulting 50Hz hum with a rejection of -130dBc which far beyond the limit of adibility even at full modulation.

 

A drop of the mains line voltage at such performances means nothing to the behold BPA768 due to the integrated power conditioner. A very low voltage drop when switching on this amplifier will not result in flickering the room light.




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